fs.CountError is called when an error is encountered. The method was
calling GlobalStats().Error(err) which incremented the error at the
global stats level. This led to calls to core/stats with group= filter
returning an error count of 0 even if errors actually occured.
This change requires the context to be provided when calling
fs.CountError. Doing so, we can retrieve the correct StatsInfo to
increment the errors from.
Fixes#5865